Sugar repression of gene expression is a ubiquitous regulatory mechanism for both prokaryotic and eukaryotic cells to adjust to changes in nutrient availability. In multicellular plants, expression of enzymes involved in carbohydrate metabolism is often feedback-regulated by sugar metabolites (Sheen, J., Photosynth. Res., 39:427-438, 1994). For instance, carbohydrate depletion has been shown to enhance expression of a wide variety of genes involved in photosynthesis, reserve mobilization, and export processes (Koch, Annu. Rev. Plant Physiol. Plant Mol. Biol., 47:509-540, 1996).
.alpha.-amylases are the major amylolytic enzymes for hydrolysis of starch stored in the endosperm during germination of cereal grains. These enzymes are encoded by a group of genes whose expression is repressed by sugars. In the embryos of germinating rice seeds, the transient expression of certain .alpha.-amylase genes is thought to be induced by sugar depletion in the embryos following imbibition of the seeds, and suppressed by sugar influx from the endosperm as germination proceeds (Yu et al., Plant Mol. Biol. 30:1277-1289, 1996). Similarly, expression of .alpha.-amylase genes in cultured rice suspension cells has also been shown to be suppressed by sugar present in the medium and induced in its absence (Yu et al., J. Biol. Chem., 266:21131-21137, 1991, Gene 122:247-253 1992; Chen et al., Plant J., 6:625-636, 1994). Further studies reveal that the transcription rate of .alpha.-amylase genes in rice suspension cells increases in response to sucrose depletion in the culture medium (Sheu et al., Plant J., 5:655-664, 1994). Protein expression systems have been established to make use of the sugar-responsive promoters of these .alpha.-Amylase genes (U.S. Pat. No. 5,460,952 to Yu et al.).
Sugar-regulated stability of rice .alpha.-amylase mRNAs, on the other hand, has also been investigated. Under transcription-inhibiting conditions, the mRNA half-lives of three .alpha.-amylase genes were found to be prolonged by sucrose-starvation (Sheu et al., J. Biol. Chem., 271:26998-27004, 1996). This result suggests that, in addition to enhanced transcription rate, increased mRNA stability may also contribute to the elevation of the steady-state levels of certain .alpha.-amylase mRNA species in sucrose-starved cells. It is unclear, however, what mechanism is responsible for the increased degradation of .alpha.-amylase mRNAs in cells provided with sucrose as compared to cells deprived of sucrose. It has been postulated that mRNA transcripts are selected for degradation in a sequence-dependent manner (Sachs, Cell, 74:413-421, 1993). Yet it remains largely unknown what sequences in RNA transcripts are responsible for targeting degradation in plants.